As electronic apparatuses have become smaller in size and multi-functional, a higher density has been required for circuit boards used for the electronic apparatuses.
Ordinary means for achieving a higher density of circuit boards include fabricating circuit boards into a multi-layer structure, as is well known in the art. The multi-layer circuit board is generally fabricated by stacking an electrically insulating layer (2) on an internal layer circuit board including an electrically insulating layer (1) and a conductor circuit (1) formed on a surface of the electrically insulating layer (1), forming a conductor circuit (2) on the electrically insulating layer (2) and if necessary, further stacking the electrically insulating layers and the conductor circuits in several stages.
When wiring is formed at a high density in a multi-layer construction as described above, exothermy occurs in the circuit board and in an electronic device. The electrically insulating layer can use a material blended with a halogen-type flame retardant as described in Japanese Unexamined Patent Publication No. 2-255848, for example, to impart flame resistance. The material blended with the halogen-type flame retardant generates halogen-type detrimental substances when it is burnt. Due to problems of environmental destruction and the greenhouse effect, however, the use of halogen-type flame retardants has been restricted. Therefore, a material providing flame resistance equivalent to that of the flame retardants of the prior art has been desired.